Common names: ITU-TSS (CCITT) X.21, ISO 4903
 (at the DTE)
 (at the DTE)
 (at the DCE)
 (at the DCE)
CORRECT?
15 PIN D-SUB MALE at the DTE (Computer).
15 PIN D-SUB FEMALE at the DCE (Modem).
| Pin | Name | Dir | Description | Type | 
|---|---|---|---|---|
| 1 | n/a |  | Shield | Ground | 
| 2 | T (A) |  | Transmit (A) | Data | 
| 3 | C (A) |  | Control (A) | Control | 
| 4 | R (A) |  | Receive (A) | Data | 
| 5 | I (A) |  | Indication (A) | Control | 
| 6 | S (A) |  | Signal Timing (A) | Timing | 
| 7 | B (A) |  | Byte Timing (A) | Timing | 
| 8 | G |  | Ground | Ground | 
| 9 | T (B) |  | Transmit (B) | Data | 
| 10 | C (B) |  | Control (B) | Control | 
| 11 | R (B) |  | Receive (B) | Data | 
| 12 | I (B) |  | Indication (B) | Control | 
| 13 | S (B) |  | Signal Timing (B) | Timing | 
| 14 | B (B) |  | Byte Timing (B) | Timing | 
| 15 | unused | 
Note: Direction is DTE (Computer) relative DCE (Modem).
| Contributor: | Joakim Ögren | 
| Source: | X.21 Page at Connectivity Knowledge Platform (Made IT) | 
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  Document last modified: 2002-01-13